1. Field of the Invention
The present invention relates to an image processing apparatus which calculates a brightness histogram of image data.
2. Description of the Background Art
Conventional image pickup devices such as digital still camera and digital video camera have been equipped with an image processing circuit for performing a variety of digital processes such as pixel interpolation, gamma correction, color space conversion and color suppression after A/D converting captured image data. Such kind of image processing circuit is often mounted on an image pickup device in the form of a chip (integrated circuit) for reducing the apparatus size, reducing the price, as well as for reducing power consumption. Furthermore, for improving the image quality, most of image pickup devices have a function of calculating a brightness histogram indicating the frequency count of brightness value in captured image data and performing, for example, brightness adjustment of e.g. white balance, filtering process, and image processing of the image data on the basis of the brightness histogram thus calculated. Brightness histogram operation can be executed by software processing of CPU mounted on the image pickup device, however, from the view point of executing the operation in short time and reducing the process load and power consumption of the CPU, it is general for an image pickup device to be equipped with a special-purpose integration circuit for executing brightness histogram operation apart from an image processing circuit that performs pixel interpolation and gamma correction, for example.
However, in association with that image pickup devices handle image data of higher tone, the image size of that image data and the bit length of pixel data significantly increase, and hence the number of registers and memory capacity to be mounted on the integration circuit for brightness histogram operation also increase, which posed the problems of increase of power consumption, expansion of packing area, and increase of production cost. For example, in the case of executing brightness histogram operation with respect to image data of 480×640 pixel size and 8-bit tone, the required number of registers to be mounted on the above integration circuit is 28, namely 256 at the maximum, and since the number of times of the brightness value has the potential of becoming 307,200 (=480×640) times at the maximum, it is necessary to prepare a storage area of 19-bit length. Furthermore, in the case of executing brightness histogram operation with respect to image data that has an image size of 2 mega (=2×10242) and 12-bit tone, the maximum of 212, namely 4096 registers to be mounted on the above integration circuit are required, and the number of times of brightness value has the potential of becoming 2,097,152 (=2 mega), so that a memory device of as much as 21-bit length is required.